The present invention relates to small antenna structures. The invention relates particularly to internal antennas that are used in mobile stations and that are fed from one feed point.
Particularly the increasingly diminishing size of mobile stations sets new requirements of diminishing the antenna structures used in the devices. However, the size of an antenna depends on the principles of physics, since the bandwidth of antenna resonance depends on the Q value of the antenna structure such that the lower Q value an antenna has, the wider is the available bandwidth. The easiest way to lower the Q value of an antenna is to make the antenna larger, but if the space required by the antenna is limited, it is extremely difficult to lower the Q value.
An advantage of planar inverted F antennas (PIFA) is their small size, allowing them to be integrated into a device so that they are entirely disposed inside said device. FIG. 1a shows a prior art conventional PIFA antenna element 100, the antenna element 100 comprising a planar radiator 110, a ground plane 120, a ground point 102 and a feed point 101. The length of edges 104 and 105 of the radiator 110 is 40.0 mm, the length of edges 107 and 108 is 25.0 mm, and the feed point is located at a 2.0-mm distance from both edge 108 and edge 104. The width of the grounding line of the ground point 102 is 5.0 mm and it is located at the edge 104, so that a centre parallel to the edge 104 of the grounding line is located at a 12.5 mm distance from the edge 108. The length of edges 121 and 122 of the ground plane is 100.0 mm, the length of edges 123 and 126 is 40.0 mm, and the distance between the ground plane 120 and the radiator 110 is 5.0 mm. Either air or another dielectric material is provided as insulating material between and on top of the ground plane 120 and the radiator 110. The radiator 110 of the PIFA antenna is coupled to the ground plane 120 via the ground point 102. The shape of the ground point may be dotted or similar to the grounding line shown in FIG. 1a. Below, reference 102 denotes the ground point and the grounding line. The physical dimensions of the radiator 110 and the ground point 102 and the distance between the radiator element and the ground plane affect the resonance frequency of a PIFA antenna. The radiator 110 is fed either from the edge of the radiator or by conveying a feeder line through the ground plane and the insulating material as FIG. 1a shows. A change in the width of the grounding line of the ground point 102 causes a change in the resonance frequency of the antenna. A decrease in the width of the grounding line causes a decrease in the resonance frequency; similarly, a wider grounding line increases the resonance frequency. The grounding line may be either as wide as the side of the antenna element or, at its narrowest, only a conductor.
The major problem in PIFA antennas is a narrow impedance band, resulting mainly from the distance between the radiator of the antenna and the ground plane with respect to the wavelength.
FIG. 1b illustrates the frequency band of the antenna structure of FIG. 1a using the above dimensions. In the graph, the x-axis shows frequency in GHz and the y-axis the radiation efficiency of the antenna element [%], antenna efficiency [%] and antenna matching (S11) [dB]. FIG. 1b shows that the frequency band of the antenna structure of FIG. 1a, at 50% antenna efficiency, is in the range of about 1400 to 1700 MHz.
The radiation efficiency of an antenna element refers the radiation efficiency of the antenna element when the antenna is matched. Antenna efficiency refers to the efficiency of the antenna when the efficiency includes antenna matching.
Attempts have been made to increase the bandwidth of a PIFA antenna for example by creating parallel resonators in the antenna structure. FIG. 2a shows an antenna structure, wherein resonances are generated with two antenna elements 201 and 202 of slightly different lengths, of which the smaller element 202 generates a higher frequency resonance and the larger element 201, a lower frequency resonance.
FIG. 2b shows an antenna structure having a main element 205 and a parasitic element 206, the elements 205 and 206 being separated from one another along the entire length to generate resonances. However, the increase in the bandwidth of the above antennas remains relatively small compared with the bandwidth created by the antenna of FIG. 1a. 
An arrangement of several adjacent resonances is a way to increase the bandwidth of an antenna. Matching of an antenna element may provide the adjacent resonances. Matching can be carried out for example with a feed and grounding strip, allowing the impedance of the strips to be arranged as desired by means of dimensioning the width and length of the strips and by means of the relationship between the mutual distances between the strips. Resonances provided with matching are easily lossy, which may result in a loss of the gain achieved by matching.
In the solution carried out on the antenna element, grooves are added to the antenna element to increase the number of resonance frequencies. However, grooves easily act as groove radiators in small antennas, making adjacent resonating antenna elements couple strongly to one another providing a resonator around the groove. This further results in the radiation resonance being low at said frequency and current densities being high in the vicinity of the groove, increasing the losses of the antenna.
The Applicant""s earlier European patent application 1 020 948 discloses a dual band antenna structure having a first groove for providing resonance in the higher 1800 MHz frequency range. The radiator also comprises a second groove that branches from said first groove. Increasing the width of the second groove decreases the bandwidth in the GSM 1800 MHz frequency range and decreases the amplification of the resonance element in the GSM 900 MHz frequency range. Increasing the length of the second groove increases the bandwidth in the GSM 900 MHz frequency range and decreases the amplification in the 1800 MHz frequency range. In said antenna structure, said second groove provides an increase in bandwidth in the lower frequency range (900 MHz) and a decrease in the higher frequency range (1800 MHz). This kind of solution is thus not well suitable for use in cases when the attempt is to accomplish as wide a bandwidth as possible for the upper frequency range.
An antenna structure is now provided for use particularly, but not necessarily, in mobile systems, the implementation allowing the Q value of an antenna to be lowered, thereby causing an increase in its bandwidth. A feed point and a ground point, arranged in the radiator of the antenna structure, the radiator comprising a planar electrically conductive surface, are separated from one another with a groove that is arranged in the planar radiator such that a line segment, to be provided between the feed point and the ground point, cuts the groove. The smaller portion of the groove is provided on the side of the line segment cutting the groove comprising the open end of the groove, and, correspondingly, the larger portion of the groove is provided on the opposite side of said line segment. The addition of a groove of the type described above to a radiator results in a change in some paths of surface currents distributed to the surface of the radiator, causing the antenna to generate a plurality of resonances and increasing the bandwidth at good radiation efficiency. The substantial length of the groove exceeds a quarter of the wavelength of the highest resonance frequency. The length is defined as the straightest possible path within the groove between the starting and end points. The starting point of said path is located in the middle of the open end of the groove. The end point is located at that point of the edge of the radiator within the groove, to which the distance of the straightest possible path within the groove, measured from the starting point to said end point, is at its longest.
The groove provides an open space in the middle area of the antenna, thereby also decreasing the capacitive coupling of the different antenna element parts. A further advantage is that the space used by the antenna is utilized as efficiently as possible.
A first aspect of the invention provides an antenna structure comprising a ground plane, a radiator located at a distance from the ground plane, an insulating layer between said ground plane and said radiator, at least one feed point for feeding a signal to said radiator, at least one ground point for grounding the radiator to the ground plane, in that the radiator comprises at least one groove comprising an open end and a closed end, the groove being arranged at least partly between said at least one feed point and said at least one ground point such that a line segment to be created between said feed point and said ground point cuts said groove, whereby a smaller portion of the groove is arranged on that side of the line segment cutting the groove, on which the open end of the groove is provided, and a larger portion of the groove is provided on the opposite side of the line segment cutting the groove, on which side the closed end of the groove is arranged.
A second aspect of the invention provides a radio device comprising an antenna structure for transmitting a radio-frequency signal, the antenna structure further comprising a ground plane, a radiator arranged at a distance from the ground plane, an insulating layer between said ground plane and said radiator, at least one feed point for feeding a signal to said radiator, at least one ground point for grounding the radiator to the ground plane, in that the radiator comprises at least one groove comprising an open end and a closed end, the groove being located at least partly between said at least one feed point and said at least ground point such that a line segment to be created between said feed point and said ground point cuts said groove, whereby a smaller portion of the groove is arranged on that side of the line segment cutting the groove, on which the open end of the groove is provided, and a larger portion of the groove is provided on the opposite side of the line segment cutting the groove, on which side the closed end of the groove is arranged.